S100/110 vs G12/G15

I'll speak to IQ and optics. Significant differences in IQ performance at tele. The achievable image quality on the S series is limited by the maximum input pupil of the small lens. The G15 has 3.3x greater area at tele end - 1.7 stops. That's more light getting to the same size sensor which you can put towards shorter shutter, lower ISO & noise, or lower scene luminosity.

The smaller lens on the S series also puts greater requirements on quality optics with more stringent alignment tolerances. Both Canon and Sony have shown fabrication yield and QC issues on their small lens cameras.

I was a big fan of the S series years ago, but given that their utility for me rapidly drops with zoom, my personal choice is the use my iPhone and iPad as my "always with me" camera and go for the G series for quality (especially the G1X).